Some electric acoustic converters include an acoustic element composed of an acoustic resistance member. In particular, microphones require acoustic resistance members having appropriate acoustic resistance values in order to obtain flat frequency response and directional characteristics. Examples of usable acoustic resistance members include materials having appropriate breathable characteristics such as clothes, unwoven clothes, and sponges. Unfortunately such acoustic resistance members have uneven aperture sizes; hence, acoustic resistance values of these materials have variations. This precludes the uniformity of acoustic resistance values and ready designing of the acoustic resistance values of the acoustic resistance members. Conventional audio equipment must be produced in consideration of variations in the acoustic resistance values of the acoustic resistance members. Setting the uniform acoustic characteristics of the audio equipment with taking each variation in the acoustic resistance values of the acoustic material into account is a major factor in high cost.
Taking such a situation into account, acoustic resistance members described in Japanese Patent Laid-open Application Publication Nos. S59-38800 and S59-39195 each have a double-layered structure which includes a filler sheet (hereinafter referred to as “filler portion”) placed on a part of a mesh portion, which takes on acoustic impedance. The filler portion functions as an air-shielding layer which does not allow acoustic waves to pass through. The mesh portion functions as an acoustic resistance layer which allows acoustic waves to adequately pass through. The air-shielding layer has an opening to expose a part of the acoustic resistance layer. The acoustic resistance layer has even areas between the threads of the mesh. In other words, the acoustic resistance layer has even areas through which acoustic waves pass. Additionally, a part of acoustic resistance layer overlaying the filler portion blocks out acoustic waves. Namely, with the acoustic resistance members described in Japanese Patent Laid-open Application Publication Nos. S59-38800 and S59-39195, the exposed portion of the acoustic resistance layer is formed by making the opening in the air-shielding layer.
In this way, the acoustic resistance members described in Japanese Patent Laid-open Application Publication Nos. S59-38800 and S59-39195 can define an acoustic resistance value from the area of the opening. Thus, the invention described in Japanese Patent Laid-open Application Publication Nos. S59-38800 and S59-39195 can provide the acoustic resistance members having uniform acoustic resistance values.
While the acoustic resistance members described in Japanese Patent Laid-open Application Publication Nos. S59-38800 and S59-39195 can define an area through which acoustic waves pass, the opening portion of the air-shielding layer is made through complicated processes involving, for example, a photographic technique, or piercing. Thus, the acoustic resistance members described in Japanese Patent Laid-open Application Publication Nos. S59-38800 and S59-39195 often have disadvantages of low processing accuracy on making the openings, and thus variations in the areas of the openings. The variations in the areas of the opening in the acoustic resistance members described in Japanese Patent Laid-open Application Publication Nos. S59-38800 and S59-39195 lead to disadvantageously large variations in the acoustic resistance values between individual products. In addition, the acoustic resistance members described in Japanese Patent Laid-open Application Publication Nos. S59-38800 and S59-39195 cannot be readily processed. Thus, the methods for making the acoustic resistance members described in Japanese Patent Laid-open Application Publication Nos. S59-38800 and S59-39195 require many processes and long hours, which disadvantageously causes high cost for making the acoustic resistance members.